The functions of a power system operating under stored program control are directed by a system stored program that operates in real time and specifies the various operating and testing functions performed. This program known as firmware is normally stored in a type of memory called a ROM or EPROM memory device which is included in a microcomputer dedicated to the particular power system. Should it be desired to add new options or capabilities to the basic power system, the overall system stored program must be changed. This may be readily accomplished by substituting a new ROM device containing a new program in newly manufactured power systems; however, it is not accomplished so readily on already existing systems in the commercial marketplace that are not readily available for the insertion of new ROM memory devices.
The addition of new options by changing the primary stored programs may further be complicated by the fact that a modification or addition to one portion of the program may cause an inadvertent change in another portion of the program; thereby, converting a formerly flawlessly operating power system program into a bug ridden malfunctioning demon.
Since many features of a power supply system will be changed and added during the life of the product, it is desirable to provide for ready modification and addition of features or options without requiring changes to the original stored control program.